Here's what I know:
Humor requires patience. The punch line is almost always the last thing to be said, and yet so many people want to say it first. They can't wait to get to the funny part, even though it's the waiting and the building that will make it funny.
When I describe my living circumstances in my early twenties, I say it like this:
"I lived with a family of Jehovah's Witnesses in a converted pantry off the kitchen with a guy named Rick who spoke in tongues in his sleep and the family's indoor pet goat."
A bad storyteller - or an unfunny person - always wants to get to the goat as soon as possible (because it's the funniest part) rather than building to it. They say the funny part first and then fill in the rest of the details when they no longer serve to increase the humor.
I hear this all the time. Both in regular conversation as well as storytelling onstage.
Considering all this, here is what I suspect:
People who make it a habit of interrupting other people are the least funny people I know. These are people who can't wait to speak. Can't wait to insert their voice into the conversation. Can't wait even a second to interject.
These are people who can't wait on a punchline.
But I'm not sure. It's just a theory.